We find non-zero mean radial velocities ranging from \(\sim -10\,\mathrm {km\,s}^{-1}\)  to \(\sim 20\,\mathrm {km\,s}^{-1}\) , which are dependent on the distance to the plane. The \(\langle V_R\rangle \)  decreases towards larger \(R\)  as a whole, coupled with shallowing vertical gradients. Considering different metallicity bins, there is evidence that the \(\langle V_R\rangle \)  of metal-rich stars decreases with \(R\)  more steeply than that of metal-poor stars in the range of \(8<R<12\,\mathrm {kpc}\) , which is in agreement with the finding of [1]} from the RAVE FGK turn-off stars. The observed radial flow and its alternate direction near the plane are likely generated by the wave-like spiral structures in the plane ([2]}; [3]}). [2]} argued that the effect of the spirals on the stellar radial velocities keeps strong up to \(Z\sim \pm 0.5\,\mathrm {kpc}\) . Besides, [5]} found that the central bar could produce radial bulk motions within \(R<10\,\mathrm {kpc}\)  and give rise to a negative radial gradient in the radial bulk velocities in the range from \(Z=0\)  to \(Z\sim 2\,\mathrm {kpc}\) . Within \(8<R<12\,\mathrm {kpc}\) , populations with different \(\rm {[Fe/H]}\)  respond differently to the perturbations, with the tendency for the metal-rich populations to show a larger fractional contribution to the disturber than the metal-poor ones, though all of them have consistent vertical trends. The behaviors of \(\langle V_R\rangle \)  agree with the hypothesis that the in-plane substructures, mainly including the spirals and the bar, play roles in producing the structure of radial bulk velocities in the disk. Nonetheless, we cannot exclude the possibility that an external perturber, such as the passage of a satellite, causes the wobbly radial bulk motions.
